This invention relates to a transmission to couple a vehicle engine with the vehicle drive wheels, in particular the transmission disclosed herein combines a torque converter with a plurality of planetary gear arrangements to provide a large number of gear ratios.
Transmission of power from a vehicle engine to the vehicle wheels ideally will allow the vehicle engine to operate at or near a most efficient constant speed throughout the complete range of vehicle speeds. From a practical standpoint this goal is unattainable in view of the fact that the vehicle must be started from a standing position. Furthermore, while using a geared transmission, engine speed must vary throughout the range of the particular gear set engaged. The utilization of a torque converter helps to overcome problems of initial start-up and further provides a broader torque transmission range; however, fluid drive is relatively less efficient than a mechanical drive. A mechanical drive, although more efficient than a fluid drive, requires a relatively high number of gear ratios in order to allow the engine to operate in its most efficient speed range. However, in adding more gear ratios, efficiency is sacrificed through friction losses and the like in inactive gear ratios. Accordingly, compromise transmissions incorporating features of a fluid drive or torque converter and a mechanical transmission have been designed to take advantage of the best characteristics of both transmissions.
A problem encountered in the design and construction of such combination transmissions is the attainment of uniform step ratios between the mechanical shifts. Uniform step ratios are ideally most efficient in such transmissions, as such uniform step ratios eliminate less efficient matches between engine speeds and desired vehicle speed inherent in a transmission which couples large step ratios with smaller step ratios.
An example of a combination torque converter and planetary gear arrangement is found in U.S. Pat. No. 3,347,113 issued to Ramsel on Oct. 17, 1967. Ramsel's patent, which was designed for use in scrapers of the kind used in earthmoving operations, provides six forward speed ratios of mechanical drive at high ground speeds while allowing for a fluid drive at lower ground speeds. Such fluid drive in lower speeds has been found acceptable in scraper operations. Although Ramsel's patent approaches a uniform step ratio, there still remain certain uneven steps. In order to provide more efficient operation at lower speeds required for other types of vehicles, either additional mechanical gear sets must be provided or the arrangement of the various planetary units and the sizing of the planetary elements must be modified to provide constant step ratios throughout a broad range of output speeds. U.S. patent application Ser. No. 420,063 assigned to the assignee of this application adds additional gearing but does not provide the efficient uniform step ratio in the low range.